1. Field of the Invention
The present invention relates to electronic devices, especially micro electromechanical (MEMS) devices and method of making the same. In particular the present invention relates to the field of radio frequency MEMS and more particularly the present invention relates to MEMS near-DC to RF capacitive shunt and series switches, e. g. a switchable capacitor.
2. Description of the Related Art
RF-MEMS switches offer great potential benefits over GaAs MMICs and PIN (positive intrinsic negative) diode switches for application in wireless communication systems as described by C. T. C. Nguyen, et al. , in “Micromachined devices for wireless communications”, Proc. of the IEEE, vol. 86(8), 1998, pp. 1756–1768; by J. J. Yao, in “RF MEMS from a device perspective”, J. Micromech. Microeng., vol. 10(4), December 2000, pp. R9–R38; and by G. M. Rebeiz and J. B. Muldavin, in “RF MEMS switches and switch circuits”, IEEE Microwave magazine, December 2001, pp. 59–71, each of which is incorporated herein by reference in its entirety. Prototype RF-MEMS switches display low loss (<0.4 dB), good isolation (>20 dB), low standby power consumption, excellent linearity (IP3>66 dBm), compactness and high levels of integration as discussed by J. J. Yao, in “RF MEMS from a device perspective”, J. Micromech. Microeng., vol. 10(4), December 2000, pp. R9–R38; by G. M. Rebeiz and J. B. Muldavin, in “RF MEMS switches and switch circuits”, IEEE Microwave magazine, December 2001, pp. 59–71; by Z. J. Yao, et al.in “Micromachined low-loss microwave switches”, IEEE J. of MEMS, vol. 8(2), 1999, pp. 129–134; by J. B. Muldavin and G. M. Rebeiz, in “High-isolation CPW MEMS shunt switches-Part 1: Modeling”, IEEE Trans. Microwave Theory and Techniques, vol. 48(6), 2000, pp. 1045–1052; and by H. A. C. Tilmans, et al. , in “Wafer-level packaged RF-MEMS switches fabricated in a CMOS fab”, proc. IEDM 2001, Washington, D.C., Dec. 3–5, 2001, pp. 921–924, each of which is incorporated herein by reference in its entirety.
A typical build-up of a RF-MEMS capacitive switch in a shunt configuration implemented on a CPW (CoPlanar Waveguide) line is shown in FIG. 1 and has been discussed by Z. J. Yao, et al. , in “Micromachined low-loss microwave switches”, IEEE J. of MEMS, vol. 8(2), 1999, pp. 129–134; by J. B. Muldavin and G. M. Rebeiz, in “High-isolation CPW MEMS shunt switches-Part 1: Modeling”, IEEE Trans. Microwave Theory and Techniques, vol. 48(6), 2000, pp. 1045–1052; and by H. A. C. Tilmans, et al., in “Wafer-level packaged RF-MEMS switches fabricated in a CMOS fab”, proc. IEDM 2001, Washington, D.C., Dec. 3–5, 2001, pp. 921–924, each of which is incorporated herein by reference in its entirety. The switch comprises a suspended movable metal bridge, which is mechanically anchored and electrically connected to the ground of the CPW.
To a first order, the switch can be modeled as a capacitor between the metal bridge and the signal line. In the RF-ON state the bridge is up, hence the switch capacitance is small, hardly affecting the impedance of the line. By applying a DC bias (superimposed on the RF signal) the bridge is pulled down onto the dielectric, the switch capacitance becomes high and the switch is OFF or in the isolation state. An important figure of merit quantifying the RF performance is the down/up capacitance ratio, Cdown/Cup, which is preferably as high as possible. This ratio can be approximated by                                                         C              down                                      C              up                                ≈                                                    ɛ                0                            ⁢                              ɛ                r                            ⁢                                                A                  overlap                                                  d                  diel                                                                                    ɛ                0                            ⁢                                                A                  overlap                                                  d                  air                                                                    =                              ɛ            r                    ⁢                                    d              air                                      d              diel                                                          (                  eq          .                                          ⁢          1                )            
where dair and ddiel are the thickness of the air gap and the dielectric, respectively, ∈r is the dielectric constant of the dielectric and Aoverlap is the overlap area of the bridge and the signal line. For a given technology, as Aoverlap cancels in (eq. 1), the isolation determines the insertion loss and vice versa. The design freedom is thus constrained considerably.
A second problem encountered in capacitive switches of the type shown in FIG. 1 is the degradation of the effective down capacitance as a result of surface roughness preventing intimate contact between the beam and the dielectric, which is discussed by J. B. Muldavin and G. M. Rebeiz in “High-isolation CPW MEMS shunt switches-Part 1: Modeling”, IEEE Trans. Microwave Theory and Techniques, vol. 48(6), 2000, pp. 1045–1052. Solutions commonly pursued to attain a large down capacitance are aimed at keeping the roughness of the bridge and of the dielectric layer very low, e. g., <5 nm, and to keep the surface free from residues. Muldavin et al. and Yao et al. introduced thin bottom metals in an attempt to reduce the roughness. In particular, Z. J. Yao et al. described in “Micromachined low-loss microwave switches”, IEEE J. of MEMS, vol. 8(2), 1999, pp. 129–134, the use of a thin refractory metal layer (e. g., W). All these measures however lead to a high series resistance and hence to an increased insertion loss for a shunt switch. Obviously, in a standard design as the one shown in FIG. 1, a difficult compromise must be made as measures for improving the isolation directly lead to a deterioration of the insertion loss.
In PCT patent application WO 02/01584, “Capacitive Micro electromechanical switches” by R. York et al., a micro electromechanical switch (FIG. 2) is disclosed comprising a bottom electrode 1, a dielectric layer 2 disposed on the bottom electrode 1, a metal cap (not shown) disposed on the dielectric layer 2 and a bridge 3 disposed proximate to the metal cap such that an electrical potential applied between the bridge 3 and bottom electrode 1 causes the bridge 3 to deform and contact the metal cap. The deformed bridge is depicted with reference number 4. A problem with this device is that charging of the metal cap will reduce the force exercised on the bridge 3, which might bounce back into the original position, hereby disturbing the normal working of the switch.